The present invention relates to the manufacturing of a squeeze cast, fiber bundle reinforced article, such as a connecting rod for an internal combustion engine.
Many component parts of internal combustion engines have heretofore been made of various types of steel. It has been a recent desire that these parts be replaced by ones of a light metal to reduce the overall weight of the engine. One example of such a replacement and the attendant problems encountered would be a piston connecting rod.
Connecting rods must have at least a certain compressive or buckling strength, particularly in the rod portion thereof, yet still fit within certain tightly restrained dimensions. Conventional steel alloy rods are relatively slim in the direction of rotation in order to not come into contact with the piston skirt. In attempting to make a connecting rod out of an aluminum alloy, sufficient buckling strength must be retained and the rod must be able to fit within the necessary locus of space defined by the piston, the crankshaft, and the crankcase.
It has been discovered by the present inventors that the necessary buckling strength can be imparted to the connecting rod made of a light metal alloy by providing a reinforcing bundle of uni-directional fibers in the core of the rod portion. The light metal alloy fills in the interstices in the bundle to form a metal-fiber matrix.
However, stay within the necessary locus of space, it has been discovered by the present inventors that it is necessary to reorient the position of the ribs of the rod portion of
the connecting rod to a position at right angles to that of a conventional steel rod.
That is, the rod portion must be designed such that the second moment of area Ix about an axis X which is perpendicular to the longitudinal axis of the rod and parallel to the direction of its rotation is larger than the second moment of area Iy about an axis Y which is perpendicular to the X-axis and the longitudinal axis of the rod. EQU Ix&gt;Iy
The present applicant has proposed previously two different types of connecting rods which are made of a light alloy having rod portion 1 reinforced by a bundle of unidirectional inorganic fibers F. The connecting rod includes a smaller annular shaped end portion and a semi-annular shaped larger end portion at the ends of the rod portion. The smaller annular portion can be considered to have a center axis perpendicular to the direction of rotation of the rod when in use. The rod portion can be considered to have a longitudinal axis. The Y-axis is defined as parallel to the central axis and perpendicular to the longitudinal axis. The X-axis is defined as perpendicular to both the center axis and the longitudinal axis. In these previous designs, the second moment of area of the core about the Y-axis (I(c) y) is generally equal to or greater than the second moment of the core of area about the X-axis (I(c) x). In the embodiment shown in FIG. 5 (I(c) y) is larger than (I(c) x). As shown in FIG. 6, the second moment of area of the core about the X-axis is equal to the second moment of area of the core about the Y-axis.
When the second moment of area of the core about the Y-axis is greater than the second moment of area of the core about the X-axis, sufficient buckling strength is given to the rod portion of the connecting rod. In this manner, a fiber bundle reinforced light alloy connecting rod gives a comparable performance equal to that of a connecting rod being made entirely of steel or the like.
However, certain difficulties are encountered in casting such a connecting rod. That is, when the second moment of area of the core about the Y-axis is larger than the second moment of area of the core about the X-axis (I(c)y&gt;I(c) x), the fiber bundle is generally shaped as an ellipse having its major axis along the X-axis. As noted above, Iy of the rod must be kept less than Ix of the rod. This requires that the bundle divide the rod-shaped portion into two sections forming the side ribs of the connecting rod. When a light molten metal alloy is cast into a mold to form the matrix with the fiber bundle, a chill surface is easily formed at the boundary between the unidirectional bundle of inorganic fibers and the cavity of the mold. Often, the bundle actually contacts the side wall of the cavity. As a result, a cold shut is apt to occur at the chill surface when the connecting rod is being cast thereby creating a defect in the rod. This "cold shut" type of defect occurs when there is an insufficient gap between the bundle and the side wall of the mold cavity to permit the molten metal to flow and close around the outer surface of the bundle. A ridge or valley is created at the edge where the metal ceases to flow.
In the case of the uni-directional bundle of inorganic fibers having a circular shape as shown in FIG. 6, wherein the second moment of area of the core about the Y-axis equals the second moment of area of the core about the X-axis, (I(c)y=I(c)x, the gap formed between the uni-directional bundle of inorganic fibers F and the cavity within the mold is very small. Thus, the molten metal may chill in this gap more easily which could cause a cold shut in filling the molten metal into the mold.
To form the uni-directional bundle of inorganic fibers, stainless steel fibers or other metallic fibers or non-metallic fibers such as silicon carbide, carbon, alumina, and the like could be used. It would be of great advantage to use non-metallic organic fibers having a lower specific gravity than metallic fibers in order to produce a composite connecting rod which is light in weight. However, certain problems occur in attempting to use solely non-metallic inorganic fibers in that it is difficult to keep the bundle tightly together in the casting process because it is difficult to have the fibers adhere to one another. Further, in order to produce a reinforced article or connecting rod having a light metal alloy formed into a matrix with a bundle of uni-directional fibers, it would be desirable to pre-heat the bundle of fibers prior to the casting steps and to have the bundle of fibers retain a certain amount of the heat so that no chilling of the molten metals being cast would occur upon contact with the bundle. The usual non-metallic inorganic fibers have a low thermal conductivity. Therefore, it takes a considerable length of time to heat the bundle properly so that it will retain sufficient heat to provide good filling performance by preventing chilling of the casting metal. Further, since most non-metallic inorganic fibers have very low co-efficients of thermal expansion compared with the metal to be formed into the matrix, a certain amount of residual stress in the matrix will be retained after the connecting rod has been produced because the fiber bundle does not contract as much as or as fast as the surrounding metal.